Studies of optical devices and optical circuits, which are formed on a silicon substrate, more specifically on an SOI (Silicon on Insulator) substrate, are conducted actively, and a field called silicon photonics is created. Since the light emission and absorption coefficients of silicon are smaller than the light emission and absorption coefficients of III-V compound semiconductors, which are practically used, the introduction of germanium of excellent integration with silicon is zealously attempted (Patent literatures 1 and 2, for example).